Cavitation involves the formation of vapor bubbles in a liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial (ortransient) cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in control valves, pumps, propellers, impellers, and in the vascular tissues of plants. Since the shock waves formed by inertial cavitation are strong enough to significantly damage moving parts, cavitation is in many situations considered to be an undesirable phenomenon. It is specifically avoided or mitigated in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics.
In certain conditions, the rapid collapse of the vapor bubbles heats the interiors of the microscopic bubbles to high temperatures, which can cause the emission of light, significantly, in the ultra-violet range (see, Putterman “Sonoluminescence: Sound into Light” (February 1995) Scientific American 32); and can initiate chemical reactions, notably oxidation-reduction reactions. It has been found that cavitation can be used to treat water (e.g.: U.S. Pat. No. 5,432,756; “Zebra mussel (Dreissena polymorpha) and other aquatic organism control”, Bryden, 11 Jul. 1995; and Karpel Vel Leitner et al., “Generation of active entities by the pulsed arc electrohydraulic discharge system and application to removal of atrazine” (2005) 39 Water Research 4705).
In addition to cavitation caused by mechanical/physical agitation, cavitation can be induced with acoustic systems and electrohydraulic discharge systems.
Electrohydraulic discharge systems, also referred to as “sparkers” and “plasma guns”, were originally developed for use as a high-energy underwater acoustic source for ocean sub-bottom imaging (e.g., see Canadian Patent 1,268,851, METHOD AND APPARATUS FOR GENERATING UNDERWATER ACOUSTICS. Clements et a., 8 May 1990). The arc created between submersed electrodes, powered by a main capacitor bank, forms a steam/plasma bubble. This bubble launches a powerful acoustic pulse that can penetrate sub-bottom strata. The pressure pulse has the required intensity and wide sonic bandwidth to allow detection of return sound echoes for acoustic sub-bottom imaging.
Some of the prior-art electrohydraulic discharge systems utilize fixed opposed underwater electrodes. As the electrode material tends to be consumed with each arc discharge, this configuration only allows for short operating times before electrode replacement. As well, consumption of the fixed electrodes alters the acoustic pulse intensity and the acoustic source pulse width characteristics. As a result, electrohydraulic discharge systems with fixed electrodes provide changing output or inconsistent characteristics and thus variable effectiveness, efficiency and efficacy when applied as a pulse shockwave source generator for inducing cavitation in liquids as a means of treating the liquids.
U.S. Pat. No. 5,432,756, Bryden discloses electrohydraulic discharge systems utilizing aluminum or aluminum alloy electrodes in the form of aluminum wire drawn from a spool and a feed mechanism comprising cooperating drive wheels. Feeding electrodes and such feed mechanisms are well known in the metal fabrication industry, for example, robotic electrode-motor-feed arc welders. With such welding applications, the electrode material is relatively rapidly consumed and minor imperfections in the electrode-end geometry are essentially self correcting. By contrast, the electrodes in an electrohydraulic discharge system are more slowly consumed and it has been found that the known feeding-electrode systems often produce persistent distortions in the electrode-end geometry (for example, it is not uncommon for the electrode ends to assume a wedge or chisel shape) reducing their arc generation effectiveness.
The known power systems for electrohydraulic discharge systems suitable for use in water treatment, typically comprise a storage circuit (e.g. a capacitor or capacitor bank), connected to: a charger (e.g. a step up transformer and rectifier assembly, conventionally connected to a 120/240 volt mains supply); and, in series with the arc generating electrodes, a high voltage-high current switch (e.g., a conventional air gap switch).
The storage circuits used in electrohydraulic discharge systems suitable for use in water treatment preferably have a capacity as high as possible. Cryogenically cooled superconducting field coils have been suggested (U.S. Pat. No. 5,432,756, Bryden), but the known electrohydraulic discharge systems have utilized capacitors. The standard prior-art type of high voltage capacitors used for high-energy pulse power applications are constructed with two aluminum foil separator dielectrics, consisting of oils, films and papers. These components are wound into spools with each isolated foil extended past both ends of the cartridge assembly. The inside top terminal connections are made to the top and bottom of the cartridge. This is accomplished by a connecting solder terminal wire or bus bar, heated to a molten state in order to flow over and work between the extended layers. This foil connection method is the accepted standard for oil capacitor assemblies. The multi contacts established across the ends of the cartridge produces low inductive losses for the capacitor, but this foil-connection configuration is labour intensive to manufacture and does not provide an ideal foil connection in that solder does not adhere well to aluminum.